Effects of peptide cyclization on the interaction with oppositely charged microgels

The effect of peptide cyclization on the interaction between antimicrobial peptides and oppositely charged poly(acrylic acid-co-acrylamide) microgels of various charge density was investigated for linear and cyclic variants of peptide oligomers (C(ARKKAAKA)nC) (n = 1, 1.5, 2, 3). Through this, peptide length could be varied without substantially affecting peptide charge density and mean hydrophobicity. Furthermore, the peptides were demonstrated to display random coil conformation both in aqueous solution and when bound to oppositely charged microgels, allowing effects of cyclization to be monitored without interference from conformational transitions. With increasing peptide length, both cyclic and linear peptide variants displayed increased binding affinity to oppositely charged microgels. For all peptide lengths, however, the difference between cyclic and linear peptide variants was marginal at most, hence cyclization had little or no influence in peptide incorporation to oppositely charged microgels. In parallel, microgel deswelling increased with peptide length for both linear and cyclic peptide variants, while linear and cyclic peptide variants of the same length displayed very similar peptide-induced deswelling. Also electrolyte-induced peptide desorption from the microgels was similar for linear and cyclic peptide variants. Taken together, these findings demonstrate that end-to-end cyclization does not markedly affect peptide incorporation into, and release from, oppositely charged microgels. This opens up opportunities for the use of microgels as carriers for peptides which have been cyclized in order to improve their proteolytic and chemical stability, or in order to achieve other therapeutic advantages compared to the corresponding linear peptide variant.